Storage cells (hereinafter referred to as cells) such as secondary batteries and capacitors are often used to be connected in series. When the cells are connected in series, voltages at both ends of the series-connected cells increase. In view of this, when the series-connected cells are assumed as a single module, that is, a single package, a voltage booster type of charging circuit using an inductor or the like is necessary to charge the series-connected cells by a low-voltage power supply. A charging circuit 503 in FIG. 109 is a conventionally known charging circuit for series-connected cells.
The charging circuit 503 includes a coil (an inductor) L1, a switch S1, a switch S2, and a charging control circuit Control 5, and one end of the coil L1 is connected to an input terminal 501, one end of the switch S1 is connected to an output terminal 502, and one end of the switch S2 is connected to a reference voltage terminal. Another end of the coil L1 is connected to the other end of the switch S1 and the other end of the switch S2. Further, the charging control circuit Control 5 controls on and off of the switch S1 and the switch S2. Further, series-connected cells are connected to the one end of the switch S1.
The switch S2 is turned on by the charging control circuit Control 5. Next, an input voltage Vin is input from the input terminal 501, and a charging current to charge the series-connected cells with electric charge is stored in the coil L1.
Then, the switch S2 is turned off and the switch S1 is turned on by the charging control circuit Control 5. After that, the series-connected cells are charged with electric charge by the charging current charged in the coil L1.
The series-connected cells can be charged in this manner.
In the meantime, capacitance values of storage cells differ from each other due to their production processes.
In such a case, if there is a variation in capacitance value between series-connected cells, a variation in voltage occurs between the series-connected cells after charging is performed. When the variation in voltage occurs, a voltage concentrates on one of the series-connected cells, which causes such a problem that its life is shortened. In order to solve this problem, there has been known a voltage balance correction circuit (a cell balancing circuit) for series-connected cells for equalizing voltages of respective cells in the series-connected cells to maintain voltage balance (see, for example, Patent Document 1).
A cell balancing circuit 504 in FIG. 109 is a conventional cell balancing circuit described in Patent Document 1.
The cell balancing circuit 504 includes a coil L2, a switch S3, a switch S4, a cell Cell1, a cell Cell2, and a cell-balancing control circuit Control 6. The cell Cell1 and the cell Cell2 are connected in series and have one series-connected end connected to the output terminal 502 and another series-connected end connected to a reference voltage terminal. Further, one end of the coil L2 is connected to a contact point between the cell Cell1 and the cell Cell2, one end of the switch S3 is connected to the output terminal 502, one end of the switch S4 is connected to the reference voltage terminal, and the other end of the coil L2 is connected to the other end of the switch S3 and the other end of the switch S4. Further, the cell-balancing charging circuit Control 6 controls on and off of the switch S3 and the switch S4.
The switch S4 is turned on by the cell-balancing control circuit Control 6. Among electric charge charged in the cell Cell2, a current to maintain voltage balance between the series-connected cells is flowed into the coil L2 so as to store a charging current.
Then, the switch S4 is turned off and the switch S3 is turned on by the cell-balancing control circuit Control 6. After that, the cell Cell1 is charged with electric charge by the charging current charged in the coil L2 so as to maintain the voltage balance between the series-connected cells.
As mentioned earlier, each of the charging circuit and the cell balancing circuit needs one coil.
If the charging circuit is combined with the cell balancing circuit, a balance charging circuit for series-connected cells which charges the series-connected cells and maintains voltage balance between the respective cells is attained.
FIG. 109 is entirely a view illustrating a conventional balance charging circuit for series-connected cells. The conventional balance charging circuit for series-connected cells is constituted by the charging circuit 503 for charging the series-connected cells and the cell balancing circuit 504 for maintaining voltage balance between the respective cells.
The charging circuit 503 includes a coil (an inductor) L1, a switch S1, a switch S2, and a charging control circuit Control 5, and one end of the coil L1 is connected to an input terminal 501, one end of the switch S1 is connected to an output terminal 502, and one end of the switch S2 is connected to a reference voltage terminal. Another end of the coil L1 is connected to the other end of the switch S1 and the other end of the switch S2. Further, the charging control circuit Control 5 controls on and off of the switch S1 and the switch S2.
The cell balancing circuit 504 includes a coil L2, a switch S3, a switch S4, a cell Cell1, a cell Cell2, and a cell-balancing control circuit Control 6, and the cell Cell1 and the cell Cell2 are connected in series and have one series-connected end connected to the output terminal 502 and another series-connected end connected to the reference voltage terminal. Further, one end of the coil L2 is connected to a contact point between the cell Cell1 and the cell Cell2, one end of the switch S3 is connected to the output terminal 502, one end of the switch S4 is connected to the reference voltage terminal, and the other end of the coil L2 is connected to the other end of the switch S3 and the other end of the switch S4. Further, the cell-balancing charging circuit Control 6 controls on and off of the switch S3 and the switch S4.
FIGS. 110 to 113 are views to explain operations of the conventional balance charging circuit for series-connected cells.
Firstly, the switch S2 is turned on by the charging control circuit Control 5. Next, an input voltage Vin is input from the input terminal 501, and a charging current to charge the series-connected cells with electric charge is stored in the coil L1. A path of the charging current is indicated by a dotted arrow in FIG. 110.
Then, the switch S2 is turned off and the switch S1 is turned on by the charging-amount control circuit Control 5. After that, the series-connected cells are charged with the charging current charged in the coil L1. A path of the charging current is indicated by a dotted arrow of FIG. 111.
In parallel with the afore-mentioned operation of the charging control circuit Control 5, the switch S4 is turned on by the cell-balancing control circuit Control 6. Among the electric charge charged in the cell Cell2, a current to maintain voltage balance between the series-connected cells is flowed into the coil L2 so as to store a charging current. A path of the charging current is indicated by a dotted arrow of FIG. 112.
Then, the switch S4 is turned off and the switch S3 is turned on by the cell-balancing control circuit Control 6. After that, the cell Cell1 is charged with the charging current charged in the coil L2 so as to maintain the voltage balance between the series-connected cells. A path of the charging current is indicated by a dotted arrow of FIG. 113.
In the conventional balance charging circuit for series-connected cells, the series-connected cells are charged with electric charge and the voltage balance between the series-connected cells is maintained as such, so as to obtain an output voltage Vout from the output terminal 502.